Method and apparatus for forming an elongated silicon crystalline body using a &lt;110&gt;{211}orientated seed crystal

ABSTRACT

Method and apparatus for forming an elongated silicon crystalline body using a capillary action shaping technique. The means for growing and pulling the body from the capillary of the die includes a &lt;110&gt;{211}oriented seed crystal.

BACKGROUND OF THE INVENTION

The invention relates a 21 apparatus for improving the capillary actioncrystal growth techniques for silicon.

CROSS REFERENCES TO RELATED APPLICATIONS

"Method and Apparatus for Forming Silicon Crystalline Bodies" Ser. No.677,579, T. F. Ciszek, filed simultaneous with this application,describes a method and apparatus for forming an elongated siliconcrystalline body using a specially designed capillary die. The methodand apparatus uses a higher melt meniscus in the central region of thegrowth front than at the edges of the front. The edges of the topsurface of the die are not concentric with the ribbon cross-section.

DESCRIPTION OF THE PRIOR ART

The growth of monocrystalline silicon is usually accomplished by theCzochralski or float-zoning techniques which produce solid cylindricalmonocrystals. Ribbon shaped silicon has been formed by severaltechniques. The dendritic web growth method entails the solidificationof a silicon sheet from a melt meniscus which is bounded at the ends bythin dendrites growing downward into a super-cooled melt region andbounded at the top by a ribbon-melt interface. This dendritic webtechnique can be understood in greater depth by reference to thepublications of S. N. Dermatris et al, IEEE Trans, Commun. andElectronics, Vol. 82, 94 (1963) and D. L. Barrett et al, J. Elec. Chem.Soc., vol. 118, 952 (1971). The Stepanov method utilizes a non-wettingdie, in contact with the melt, to shape the meniscus for ribbon growthas described in A. V. Stepanov, Zh, Tekh, Fiz. Vol. 29, 381 (1959) andJ. Boatman et al, Elec. Chem. Tech. Vol. 5, 98 (1967).

More recently, a capillary action shaping technique has been devised forthe growth of crystals with complex cross-sectional shapes. Thetechnique was first applied to the growth of sapphire crystals, and wascalled Edge-defined, Film-fed Growth (EFG). It has since been applied tomany other materials, including silicon. In the EFG technique, thecrystal is grown from a thin molten zone of liquid at the top surface ofa capillary die. As the crystal grows, fresh liquid is supplied from themelt reservoir in a crucible via capillary rise through channels in thedie. The outer edges of the die top bound the lower portion of themeniscus from which the crystal grows and hence determine its shape.While numerous shapes have been grown, most work has concentrated uponribbons, rods, and tubes, especially ribbons. This technique can beunderstood with reference to papers of H. E. LaBelle, Jr., MaterialsResearch Bulletin, Vol, 6, 581 (1971), U.S. Pat. No. 3,591,348, issuedJuly 6, 1971 to H. E. LaBelle, Jr., T. F. Ciszek, Materials ResearchBulletin, Vol. 7, 731 (1972), T. F. Ciszek, et al, Physical Stat. Sol.Vol. 27, 231 (1975) and J. C. Swartz et al, Journal Electron MaterialsVol. 4, 255 (1975).

Some advantages of the capillary action shaping technique include theelimination of post-growth wafering or shaping, a potential forcontinuous growth and processing, minimal solute segregation, and a highlinear growth speed. However, the technique also has drawbacks. Theseinclude the requirement for a non-reactive, insoluble, durable andwettable shaping die, critical isotherm control requirements at the dietop, generally poor crystal structure (for silicon) and a low materialthroughput rate.

During silicon elongated crystal growth from carbon dies, small siliconcarbide crystallites form in the orifice of the die. Such crystallitesare also found floating in the meniscus at the top of the die. More orless frequently such a particle becomes attached to the silicon crystalthus destroying the perfection of the crystal. Similar observations aremade on the outside of a carbon die in contact with the molten silicon.Crystallites accumulate in the orifice of the carbon die used forcrystal growth. Scanning electron micrographs of such small crystalsshow these crystals to appear equi-axed and expose mainly (111) and(100) surfaces. The morphology of these crystals is in agreement withthe growth from a liquid phase by precipitation. Some of these crystalsare found in clusters bonded to each other with a very limited areawhere their orientations are identical. It appears that the smallcrystals are grown by this technique as an unwanted "side-product" of Siribbon pulling.

Another problem in the growth of elongated silicon crystals is twinning.Twinning means an abrupt rotation of 180° of the crystal atomicstructure on a {111} crystal plane. This effect disrupts crystal growthfrom the seed crystal during the nucleation stage and results in not asingle crystal elongated silicon body but a body of polycrystallinesilicon.

Twinning is a dominant mechanism during crystal growth of siliconribbons and of great influence on the perfection of the ribbon. Thenucleation frequency of twins is very high during the initial seedingoperation and substantially lower during "steady-state" growth. Thenucleation events leading to twinning can be categorized by twinningduring seeding and twinning at surface inclusions.

SUMMARY OF THE PRESENT INVENTION

A method and apparatus for forming silicon crystalline bodies isdescribed where a <110> {211} oriented seed crystal is used. Thisorientation has been found to reduce the accumulation of crystallites inthe orifice of the die during growth. Also, the twinning disruption ofthe growth of single crystals during the nucleation phase is overcome.When twinning occurs where the original growth plane is {110}, theoriginal crystal orientation of <110> will be repeated. This allows thecontinuation of crystal growth without interruption and the undesirableformation of polycrystalline silicon. Twin boundaries when produced areparallel to the <110> growth direction.

It is possible to produce an extremely thin elongated body of silicon byuse of heat modifiers positioned above the die and along the path of thesilicon crystalline body being grown. For a ribbon structure, twoparallel metal plates closely positioned on either side of the siliconribbon allows the production of silicon ribbon which is in the order ofmicrons or thinner in thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a overall view of the crystal pulling apparatus of the presentinvention which is principally in cross-section;

FIG. 2 is a cross-sectional view of the melt, die and thermal modifiersection of the crystal pulling apparatus;

FIGS. 3A, 3B and 3C are detailed drawings of one form of the die of thepresent invention;

FIG. 4 is a schematic illustration of one form of a continuous siliconribbon growth apparatus;

FIGS. 5A and 5B is a drawing of a thermal modifier or heat shield usefulin growing thin ribbons of crystalline silicon;

FIGS. 6, 7, 8, and 9 are schematic representations of the method forgrowing elongated crystals of the present invention;

FIG. 10 is 21 21 times enlarged photomicrograph of as-grown ribbonsurface using a constant meniscus die of the prior art;

FIG. 11 is a 21 times photomicrograph of as-grown ribbon surface usingthe variable meniscus die of the present invention;

FIGS. 12 and 13 are transmission x-ray topographs of a seed crystalinterface and 15 centimeters below the interface, respectively, for theprior art seed orientation;

FIGS. 14 and 15 are transmission x-ray topographs of the seed crystalinterface and 15 centimeters below the interface, respectively, for theseed orientation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the apparatus for forming elongated silicon bodies ofthe present invention. The apparatus includes a quartz tube 10 in whichis the hot-zone of the furnace. The source of heat for this portion ofthe furnace is the RF coil 12 which is connected to the RF generator 14.Within this hot-zone is an insulator 16 and within the insulator 16 is aconductive susceptor which holds the crucible 18, the susceptor support20 and the shaft 22 on which the supported crucible is mounted. Withinthe crucible is the molten silicon 24 and the die 26. The die can bemade of carbon or silicon carbide. A crystal being grown (not shown) ispulled and grown through the central portion of the structure above thedie and is supported by the seed chuck 30 which is supported by shaft32. Just above the hot-zone of the furnace is the water-cooled purgetube assembly 40. The water jacket is cooled by water flowing into thewater jacket 42 through inlet pipe 44 and outlet pipe 46. Alsoassociated with this water cooling system is a water-cooled end sealstructure 50 which has water flowing into the cooling jacket 52 throughinlet tube 54 and out the water jacket through outlet tube 56. A similarstructure is used at the bottom of the quartz tube 10 (not shown). Theseend structures allow gas tight connections to be made to the quartz tube10 and also cool the ends of the tube. The gate valve has inert gaspurging system input tube 64 and output tube 66. The plastic bellows ofthe system 68 is partially broken as indicated and it is long in heightto give the ability of growing long silicon crystalline bodies. There isan inert gas inlet 70 at the top of the bellows.

The closed, gaseous tight system shown in FIG. 1 results in a cleanersilicon melt at the die top surface and a resulting cleaner elongatedsilicon crystalline body. The use of inert gas, such as argon, purginghas further improved the character of the silicon bodies grown withinthe apparatus. The gas sweeps away any silicon oxides or vapor ofsilicon carbide which may be contaminating the melt and limitingperfection of the crystal body being grown. The purging is accomplishedby closing the vacuum gate valve 60 by movement of its slider 62 acrossthe opening and the region below the valve is evacuated. Inert gas suchas argon is then passed through the inlet port 64 in the gate valve.When the lower chamber is filled with argon, an argon outlet to abubbler (not shown) at the bottom of the chamber is opened.Simultaneously, the plastic bellows section 68 above the gate valve isargon purged through the argon inlet 70 and outlet 66. The seed shaft 32is initially located in this section. When both sections have beenpurged an adequate length of time, and the die is at growth temperature,the argon outlet 66 above the gate is closed and the gate valve 60 isopened. Argon then flows from the inlet 70 at the bellows top and theinlet at the gate valve 64 downward through the water-cooled purge tube40 which surrounds the crystalline body to be grown. From the bottom ofthis purge tube, the graphite draw tube 41 can be supported. The ovalbore of the draw tube 41 causes the purge gas to impinge on thesolidification region at the die top. The gas then flows out the bottomof the furnace through a bubbler (not shown). After growth, the ribbonis pulled up into the bellows 68 and the gate valve 60, 62 is closed.The bellows is then opened to remove the silicon body and reseed forsubsequent growth.

FIG. 2 shows an enlarged detailed view of the hot-zone of the apparatusfor forming elongated silicon crystalline bodies. The quartz furnaceshell 10 is partially surrounded by RF coil 12 which is connected to theRF generator 14 for heating purposes. An insulator 16 within the drawtube surrounds the crucible 18 containing the molten silicon 24. Thecrucible 18 is positioned within the susceptor 19 which is in turnsupported by support 20 which is on shaft 22. The die 26 having at leastone capillary therein is positioned partially submerged in the moltensilicon 24 and has a generally truncated wedge-shaped top. The moltensilicon wets the die and moves into the capillary and by capillaryaction moves to the top surface of the die 26. The die is held in placeby holder 27. Three thermal modifiers are shown in this embodiment. Thethermal modifiers are a lower heat shield 28, upper heat shield 29 andthe topmost heat shield 34 which has vertical projections substantiallyabove its horizontal surface which extend along the path of the growingsilicon body. This topmost heat shield is only utilized where it isdesired to produce a silicon body having a thickness in the order of 10microns or thinner. In other circumstances it is not necessary toutilize this topmost heat shield 34.

The very thin silicon bodies, such as ribbons of silicon, are verydesirable for some applications such as a photovoltaic silicon solarcell. If economical production of electrical energy from such devices isto be obtained, the amount of silicon used in the device must be small.Thus thin sheets of silicon are desirable if they can be solidifieddirectly in a form so that the material is not wasted by sawing orpolishing. The topmost heat shield 34 allows the production of thinsheets of silicon down to thicknesses of less than one micron in thecentral region of silicon ribbons. The heat shield having the twoparallel vertical plates in close proximity to the central portion ofthe growing silicon ribbon allows the crystallization and pulling ofsuch a thin ribbon. The shield is typically made of molybdenum. Thedetailed structure of this heat shield 34 is shown in FIGS. 5A and 5B.

Where it is desired to form wider ribbon widths than about 1 centimeterit is necessary to be concerned about the uniformity of temperatureacross the width of the growing crystal. In the case of even widerribbons of width greater than about 4 centimeters, auxillary heating orcooling techniques are required to assure the desired temperaturedistribution across the crystal at the solid liquid interface duringgrowth. One means for assuring this is the use of segmented heatingmeans 36 which allow the application of different amounts of heat atvarious positions across the die. A second technique is to direct theflow of the inert gas in different controlled amounts to differentsegments of the liquid solid crystal interface so as to maintain thedesired growth temperature across the growing body's interface. Thislater technique, of course, is a cooling technique.

Problems which occur when using a flat top die for capillary actioncrystal growth come about primarily from the fact that the die isvirtually always made of graphite and hence slowly dissolves in theliquid silicon. The silicon then is saturated with carbon and thesaturated silicon rises up the capillary slot in the die and comes tothe top region where crystal body growth occurs. This top region is thecoolest region in the growth system. The excess carbon which is in thesaturated silicon is forced out of the solution and comes out in theform of silicon carbide crystallites which tend to collect in the topsurface of the die because the temperature is lower here than in thebulk melt. These crystallites near the top surface tend to distort themelt meniscus and make the ribbon non-uniform in its surface smoothness.Another problem that occurs is due to the close proximity of thefreezing interface to the die top. The silicon carbide particles at theinterface tend to be incorporated in the ribbon and generatedislocations and other defects.

It has been found that it is advantageous to keep the interface of thefreezing ribbon as far as possible from the die top. FIGS. 3A, 3B and 3Cillustrate one embodiment of a die structure which accomplishes this.FIGS. 3A, 3B and 3C show the capillary 71 in die 26 and the centralportion 72 which holds the die 26 together. The die top surface 73 iscurved so that it is higher at the edges 74 than in the middle. In thisway if the crystal body solid/liquid interface can be maintainedapproximately planar, then the interface is further from the die atleast in the central region. The central region is most critical forgeneration of defects in the silicon body. However, if the width of thedie top is kept constant while the die top is curved, then the fact thatthe interface is higher in the central region means that the meniscusthere is also higher and hence the meniscus would be quite thin in thecentral region. This would cause the ribbon to be very non-uniform inthickness from one edge to the other. It would be much thinner in themiddle than at the ends. So not only should the die top curve downwardfrom the ends 74 toward the central region but it must also become widerin the central region than at the ends 74. The meniscus then has a widerbase in the central region. The wider base combined with the greatermeniscus height in the central region results in a uniform thickness atthe solid/liquid interface. In summary, there are two things that areimportant to the die design, one is the curvature and the other is thewidening of the die top in the central region.

This die structure 26 is different from the edge-defined film-fedtechnique of the prior art, because in that technique the cross-sectionof the growing ribbon or other shape is concentric with the top of thedie. The cross-section of the ribbon or other shape is not concentricwith the die top using the FIGS. 3A, 3B and 3C die structure. The dietop surface is bowed out considerably in the middle at 72 and is thin atthe edges 74 but yet a uniform thickness ribbon or other shape crystalis grown.

FIG. 4 illustrates a continuous growth ribbon process wherein as theribbon is grown it is pulled and then wound on a ribbon pulling andwind-up mechanism 76 which may be driven by a motor 77. The actualribbon growth furnace 78 is similar to that shown in FIGS. 1 and 2wherein the hot-zone or growth-zone 79 is heated by typically RF heater80 with RF power thereto from a suitable RF generator from lines 81.However, other heating means can be used. Inert gas inlets 82 aredirected onto the crystal liquid-solid interface of the ribbon beingpulled and grown. The ribbon 83 is pulled through the pull port andinert gas outlet port 84 and then is flexible enough to be wound uponthe wind-up mechanism 76 as illustrated. The flow of argon out the pullport 84 prevents diffusion of air into the furnace while providing ameans of removing the ribbon. Other means of sealing the system such asa liquid seal through which the ribbon passes could be used.

The method for forming elongated silicon crystal bodies can be furtherunderstood, particularly by reference to FIGS. 6, 7, 8 and 9. Thecapillary action shaping technique, as shown in FIG. 6 has moltensilicon 85 in a crucible 86 with a partially submerged capillary die 87therein. The liquid which is going to be crystallized rises up acapillary slot 88 to the top surface of a die 87. When the liquidreaches the top of the slot it no longer continues its motion as in FIG.6. However, if a crystal 90 is dipped into the thin slot and begins topull the liquid upward, the contact angle of the liquid with the diethen changes. The contact angle is such that the liquid can spreadlaterally over the top surface of the die until the limiting edges ofthe die top surface are reached. The edges determine the lowerattachment perimeter of the melt meniscus.

The thickness of the ribbon crystal being grown in FIG. 6 when the curvetop die with variable top width is used is essentially equal to thethickness of the die top near the ends of the die. The ribbon isessentially uniform in thickness although there may be slight variationsacross it. In the typical case, the mid region of the ribbon is somewhatthinner than the edges. The edges of the ribbon are about as thick asthe edge thickness of the die top.

FIG. 7 is a cross-section of the FIG. 6 at the center of the die priorto applying the seed crystal 90. No through capillary is shown becausethis is the area that holds the two major portions of the die 87together. FIG. 8 shows the edge condition with the ribbon in place andFIG. 9 the central portion with the ribbon in place. It can be seen thateven though the top of the die is narrow at the ends and relatively widein the middle, the ribbon thickness is essentially uniform because thefreezing interface is close to the die top near the ends of the die, buthigher above the die top near the middle. The top of the meniscus isabout as wide as the bottom of the meniscus at the end areas. However,the cross-section of the meniscus near the central point of the die andribbon tapers from a wide base to a narrow top. By proper choice of thecurvature of the die top and the taper angle of the sides of the die anoptimum value for this variation of the width of the die top withposition along the die top can be obtained. A principal parameter whichenters into the design of the die is the distance which the freezinginterface is from the top of the die near the central region. Thedistance of the interface from the top surface of the die can beadjusted by varying the curvature of the die top surface. The otherparameter used to determine the thickness variation of the die along thetop surface is the taper angle of the sides of the die.

For growth of ribbons up to about 3 cm. in width, the outer edges of thetop die surface, that is those edges which bound the lower portion ofthe melt meniscus from which the ribbon solidifies can be considered tobe determined by the intersection of a vertical truncated wedge withenclosed angle - φ truncated thickness - X_(e) and width - W with ahorizontal cylinder of radius R.

The intersection is made essentially such that the cylindrical surfacecontains the short edges, X_(e), of the wedge top. The resultant die topis that of FIGS. 3A, 3B and 3C where X_(e) is the top surface thicknessat the ends, X_(m) the top surface thickness in the center, φ theenclosed angle, R the radius of curvature of top surface, W the width ofdie, and δ the difference in height from ends to center. The top of thedie thus smoothly increases in thickness from X_(e) to X_(m) anddecreases in height, by an amount δ, as we go from the die edge to thedie middle.

The objective in this design is to attain a high melt meniscus in thecentral region, since close proximity of the freezing interface to thedie top is detrimental to ribbon perfection and surface smoothness,while still maintaining the close proximity at the die ends to stabilizethe ribbon width. Furthermore, this must be achieved in a smoothtransition to facilitate the early stages of growth from seed size tofull width.

The ribbon thickness is less than or equal to X_(e). R and φ are chosento optimize the values of X_(m) - X_(e) and δ for successful ribbongrowth. These parameters are given by ##EQU1##

    X.sub.m -X.sub.e = 2 δ tan φ/2

Experimentally, values of δ ≅ 0.8 mm and X_(m) -X_(e) ≅ 0.6 mm are foundto be useful for producing smooth silicon ribbons. Thus, for example, a25.4 mm wide die was used with X_(e) ≅ 0.51 mm, R = 101.6 mm, and φ =40°. Such a die produces ribbons of < 0.5 mm thickness even though X_(m)is about 1.09 mm. Thus, the cross-section of the ribbon is not"edge-defined" by the edges of the die top. Where the width of the dieis greater than about 3 cm., the central region of height -δ and widthX_(m) is extended somewhat before beginning the narrowing and rising tothe ends of the top surface.

In the solidification of a crystal body, it is necessary to maintain auniform temperature across the crystal at the growing interface. Thermalstresses in the body results if the variation of temperature is notcorrect. For this reason, it is common to use thermal modifiers or heatshields with the slot very close to the size of the crystal body withjust a small clearance on each side. A shield of this type tends tostabilize the temperature near the freezing interface. Thermal stressesin the ribbon can be alleviated by properly controlling the temperaturegradient along the crystal. Small variations in temperature can causealso problems in freezing up the ribbon to the die. The solution to morecareful control of the profile near the die top surface in acontinuously variable way is to build a segmented heater which is inclose proximity to the top surface of the die and goes along the lengthof the die. The heater is segmented and the segments are individuallycontrolled by electrical input. It is thereby possible to contour thethermal profile at the die top.

In FIGS. 10 and 11 the central region surfaces of two ribbons are shownat 21× magnification. Both ribbons were grown from 25.4 mm wide dies.The ribbon of FIG. 10 was grown from a flat-top EFG die of 0.5 mm topthickness. The ribbon of FIG. 11 was grown using the die design of thepresent invention with R = 101.6 mm, X_(e) = 0.5 mm, and φ = 40°. Thegrowth speeds were about 2 cm/min., and both dies were made of the sametype of graphite. The ribbon of FIG. 11 had a much smoother surface asshown, and also a much lower density of silicon carbide particlesimbedded in the surface. Thus the perfection is improved and lesssurface treatment is required to use the ribbon. MOSFET and solar celldevices have been made successfully on as-grown surfaces of smoothribbons like those of FIG. 11.

One of the major problems in the growth of silicon ribbons using carbondies is twinning. Twinning disrupts the growth of single crystals duringthe nucleation phase. An example of the effect is shown in FIG. 12. FIG.12 utilizes a seed orientation <100> {110}. Similar results occur for<111> {110}, <110> {100} and other common orientations. For all theseorientations single crystal ribbons were not obtained. The dominantmechanism for preventing single crystal growth was uncontrolled twinningon all octahedral {111} planes. Possibilities of twinning for differentgrowth planes such as (100), (110), (111) are summarized in Table I. Anexplanation of the Miller indices may be found, for example, in"Introduction to Solid State Physics" by C. Kittel Second Edition 1956,John Wiley & Sons, Inc. New York, N.Y., pages 33-35.

                  TABLE I                                                         ______________________________________                                        Original Plane                                                                              Twinning Plane                                                                              New Plane                                         ______________________________________                                        ( 1 0 0 )     ( 1 1 1 )     ( 1 -2 -2 )                                                     ( -1 1 1 )    ( 1 2 2 )                                                       ( 1 -1 1 )    ( 1 2 -2 )                                                      ( -1 -1 1 )   ( 1 -2 2 )                                        ( 1 1 0 )     ( 1 1 1 )     ( -1 -1 -4 )                                                    ( -1 1 1 )    ( 1 1 0 )                                                       ( 1 -1 1 )    ( 1 1 0 )                                                       ( -1 -1 1 )   ( -1 -1 4 )                                       ( 1 1 1 )     ( 1 1 1 )     ( -1 -1 -1 )                                                    ( -1 1 1 )    ( 5 1 1 )                                                       ( 1 -1 1 )    ( 1 5 1 )                                                       (  -1 -1 1 )  ( 1 1 5 )                                         ______________________________________                                    

Twinning occurs on the octahedral planes. Four out of eight octahedralplanes are listed. Two of these planes are always parallel to each otherand thus equivalent. Continuous twinning on these planes will leadimmediately to a breakdown in single crystal structure propagated fromthe seed crystal. The possibilities are listed in Table I. It has beenfound that the most important plane to use as the solid/liquid interfaceplane is the {110} because twinning repeats the orientation <110>. Ifthe original seed interface plane is (110), then twinning on (111) isfollowed by repeating the original crystal orientation which is [110],for example as shown in Table I.

To grow ribbons with a minimum of defects, the epitaxial relationshipbetween silicon and silicon carbide must be observed. It has been foundthat silicon and silicon carbide have a perfect epitaxial relationshipfor the <110> {112} seed orientation. Only for this orientation <110>,silicon carbide planes are parallel to the {110} silicon planes whileconcentrically the {111} silicon carbide planes are also parallel to the{111} silicon planes. For any other seed orientation, the epitaxialrelationship with the silicon and silicon carbide is such that twinningon all octahedral planes will be activated. The different epitaxialrelationships are summarized in Table II.

                  TABLE II                                                        ______________________________________                                        Plane of    Plane of                                                          Si Substrate                                                                              SiC Layer  Orientation                                            ______________________________________                                        ( 0 0 1 )   ( 0 0 1 ) β                                                                         ( 1 0 0 ) β ∥ ( 1 0 0 ) Si                           ( 1 1 0 ) β                                                                         ( 0 0 1 ) β ∥ ( -1 1 0 ) Si                                     ( 1 -1 0 ) β ∥ ( -1 1 0 ) Si             ( 1 1 0 )   ( 1 1 0 ) β                                                                         ( 1 -1 1 ) β ∥ ( 1 -1 1 ) Si             ( 1 1 1 )   ( 1 1 1 ) β                                                                         ( 1 -1 0 ) β ∥ ( 1 -1 0 ) Si                                    ( -1 1 0 ) β ∥ ( 1 -1 0 ) Si             ( 1 1 1 )   ( 0 0 0 1 ) H                                                                            ( 1 1 -2 0 ) H ∥ ( 1 -1 0 )                   ______________________________________                                                               Si                                                 

For instance, take a (001) silicon plane. There are two possibleepitaxial relationships. Silicon carbide layers can grow on an (001)silicon plane in the [001] orientation and also in the [110]orientation. If they grow in the [001] orientation, there is anorientation relationship in which (100) silicon carbide planes areparallel to the (100) silicon planes. In the second case, if the siliconcarbide layer falls in the [110] orientation then the orientationrelationship is such that (001) silicon carbide is parallel to the (110)silicon plane and also the (110) silicon carbide is parallel to the(110) silicon. Considering the [111] silicon orientation, then siliconcarbide can also grow in the [111] direction and there are twopossibilities for the silicon carbide relationship as shown also inTable II. However, in a (111) silicon substrate a possibility may occurwhere the silicon carbide grows not in the cubic form, β, but in thehexagonal mode, H. The cubic mode is indicated in Table II by a β andthe hexagonal mode by an H. The preferred seed orientation for growingsilicon ribbons is <110> {112}, as given in FIG. 14 and FIG. 15. FIG. 14shows the seed crystal interface for this orientation and indicates thatsingle crystal growth with parallel twinning has been achieved. Theoptimum silicon seed crystal orientation is [011] in the pullingdirection, [011] in the direction of the die, [112] perpendicular to themajor surface of the seed and [111] perpendicular to the edge face ofthe seed. The only defects visible are due to parallel twinning on the(111) planes as described in Table I above. The parallel twins are lowenergy faults and have no influence on charge carrier generationlifetime. Therefore good solar cell efficiency is obtained even if suchtwin planes are present. Other twin planes are of higher order andtherefore they have influence on generation lifetime. FIG. 15 shows acrystal section after 15 centimeters of growth. This section alsocontains only parallel twinning. Solar cell efficiency in the presenceof parallel twinning is between 8 and 10 percent.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand detail may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. Method for growing an elongated siliconcrystalline body comprising:establishing a molten film of siliconoverlying the top surface of a carbon containing capillary die; growingand pulling said body from the film by using a <110> direction, {211}face plane, {111} edge plane, oriented silicon seed crystal whereby saidpulling being in the <110> direction with {211} face plane and {111}edge plane which results in twinning along the <110> directionperpendicular to the {211} face plane, and parallel to the {111} edgeplane of said body and which allows any carbon impurities to beincorporated into said body so as to prevent significant nucleation ofadditional defects.
 2. The method of claim 1 wherein the said die iscomposed of carbon.
 3. The method of claim 1 further comprising:pullingthe body through a heat shield which extends from a point near said diealong the path of the body so as to produce a said body in the order ofmicrons or thinner in thickness.
 4. The method of claim 3 wherein saidbody is a ribbon and said heat shield includes two parallel plates inclose proximity to said ribbon.